1. Field of the Invention
The invention relates generally to drill string telemetry. More specifically, the present invention relates to a fault diagnosis and/or identification system for a downhole drilling operation.
2. Background Art
Downhole systems, such as Measurement While Drilling (MWD) and Logging While Drilling (LWD) systems, derive much of their value from their abilities to provide real-time information about borehole conditions and/or formation properties. These downhole measurements may be used to make decisions during the drilling process or to take advantage of sophisticated drilling techniques, such as geosteering. These techniques rely heavily on instantaneous knowledge of the formation that is being drilled. Therefore, it is important to be able to send large amounts of data from the MWD/LWD tool to the surface and to send commands from the MWD/LWD tools to the surface. A number of telemetry techniques have been developed for such communications, including wired drill pipe (WDP) telemetry.
The idea of putting a conductive wire in a drill string has been around for some time. For example, U.S. Pat. No. 4,126,848 issued to Denison discloses a drill string telemeter system, wherein a wireline is used to transmit the information from the bottom of the borehole to an intermediate position in the drill string, and a special drilling string, having an insulated electrical conductor, is used to transmit the information from the intermediate position to the surface. Similarly, U.S. Pat. No. 3,957,118 issued to Barry et al. discloses a cable system for wellbore telemetry, and U.S. Pat. No. 3,807,502 issued to Heilhecker et al. discloses methods for installing an electric conductor in a drill string.
For downhole drilling operations, a large number of drill pipes are used to form a chain between the surface Kelley (or top drive) and a drilling tool with a drill bit. For example, a 15,000 ft (5472 m) well will typically have 500 drill pipes if each of the drill pipes is 30 ft (9.14 m) long. In wired drill pipe operations, some or all of the drill pipes may be provided with conductive wires to form a wired drill pip (“WDP”) and provide a telemetry link between the surface and the drilling tool. With 500 drill pipes, there are 1000 joints, each of which may include inductive couplers such as toroidal transformers. The sheer number of connections in a drill string raises concerns of reliability for the system. A commercial drilling system is expected to have a minimum mean time between failure (MTBF) of about 500 hours or more. If one of the wired connections in the drill string fails, then the entire telemetry system fails. Therefore, where there are 500 wired drill pipes in a 15,000 ft (5472 m) well, each wired drill pipe should have an MTBF of at least about 250,000 hr (28.5 yr) in order for the entire system to have an MTBF of 500 hr. This means that each WDP should have a failure rate of less than 4×10 per hr. This requirement is beyond the current WDP technology. Therefore, it is necessary that methods are available for testing the reliability of a WDP and for quickly identifying any failure.
Currently, there are few tests that can be performed to ensure WDP reliability. Before the WDP are brought onto the rig floor, these pipes may be visually inspected and the pin and box connections of the pipes may be tested for electrical continuity using test boxes. It is possible that two WDP sections may pass a continuity test individually, but they might fail when they are connected together. Such failures might, for example result from debris in the connection that damages the inductive coupler. Once the WDPs are connected (e.g., made up into triples), visual inspection of the pin and box connections and testing of electrical continuity using test boxes will be difficult, if not impossible, on the rig floor. This limits the utility of the currently available methods for WDP inspection.
In addition, the WDP telemetry link may suffer from intermittent failures that would be difficult to identify. For example, if the failure is due to shock, downhole pressure, or downhole temperature, then the faulty WDP section might recover when conditions change as drilling is stopped, or as the drill string is tripped out of the hole. This would make it extremely difficult, if not impossible, to locate the faulty WDP section.
In view of the above problems, it is desirable to have techniques for performing diagnostics on and/or for monitoring the integrity of a WDP telemetry system.